The invention relates to a conveyor system that includes an electric motor for moving the conveyor and power supply appliance for supplying power between the electric motor and the alternating-electricity source of the conveyor system. The power supply appliance includes an inverter, which comprises a rectifying bridge connected to the alternating-electricity source, and also a motor bridge connected to the electric motor. Such a conveyor system also includes a device for determining the voltage of the alternating-electricity source.
The invention also relates to a method for controlling the power in a conveyor system of the foregoing type.
In a conveyor system of the above type, power is supplied to the motor for moving the conveyor. For example, in an escalator system, power supply to the motor generally occurs by connecting the phases of the motor directly to the electricity network with mechanical switches, such as contactors.
It is also possible to connect the power supply to the motor of the conveyor system via a frequency converter. In this case the conveyor can be used at a reduced speed with the frequency converter when the loading is small, and when the loading increases the power supply can be connected directly from the electricity network, for example, by connecting the phases of the motor directly to the electricity network. U.S. Pat. No. 4,748,394 discloses this type of arrangement.
Problems are associated with the aforementioned type of arrangement. If the control of the conveyor enables changing the speed during the running of the conveyor, for safety reasons the motion of the conveyor must be monitored because a change in speed in the middle of a run could lead to a dangerous situation. For example, a sudden change in the speed of an escalator can cause passengers to fall over. The aforementioned type of dangerous situation is possible, for example, when using a frequency converter for regulation of the speed of the conveyor. Using the conveyor at different speeds often requires measurement of the speed with two different sensors. In addition, a safety arrangement is required that stops the operation of the conveyor in a dangerous situation.
Before the motor of a conveyor system that operates at reduced speed can be connected directly to the electricity network, the speed of the conveyor must first be increased so that the frequency and phase of the supply voltage of the frequency converter can be synchronized with the network. The speed regulation the conveyor and also the synchronization of the supply voltage of the frequency converter require very complex control logic. In addition, so that the speed of the conveyor can be increased to the synchronization speed during low loading of the motor, it must be possible to forecast a change in the loading of the conveyor in some way. For this reason, for example, in connection with an escalator system or travelator system, different sensors that forecast the traffic flow of the conveyor must often be positioned with the arrival route of passengers.